Rotary screw compressors of the type set forth herein comprise two rotors mounted in a working space which is limited by two end walls and a barrel wall extending therebetween. The barrel wall necessarily takes the shape of two intersecting cylinders, each housing one of the rotors. Each rotor is provided with helically extending lobes and grooves which are intermeshed to form chevron shaped compression chambers. In these chambers, a gaseous fluid is displaced and compressed from an inlet channel to an outlet channel by way of the screw compressor. Each compression chamber during a filling phase communicates with the inlet, during a compression phase undergoes a continued reduction in volume and during a discharge phase communicates with an outlet. A rotary screw compressor of this type is disclosed in U.S. Pat. No. 4,435,139.
Rotary screw compressors of this kind are often provided with valves for regulating the built-in volume ratio for the capacity of the compressor. When continuous regulation is required, slide valves are often used, however, as with other regulation needs, it is sufficient to use lift valves. Such lift valves are mounted in the barrel wall of the compressor or may be mounted in one of the end walls and in this regard, normally in the high pressure end wall.
Several solutions for controlling the capacity of screw compressors operating at a constant number of rotations have been proposed. One such solution is disclosed in U.S. Pat. No. 5,108,269 issued Apr. 28, 1992. This solution provides radially positioned valves in the side wall of the barrel with the valves being opened so as to communicate the particular compression chamber with either the inlet or outlet manifold. However, as will be discussed in greater detail hereinbelow, with such valves, compression losses due to leakage clearance valve and between the valves and the rotors are experienced to the extent that full capacity cannot be realized.
Of the above noted solutions, the use of conventional slide type valves which constitute a portion of the barrel of the compressor has the advantage of providing a wide control range and the possibility that at a constant working pressure ratio in the compressor a relatively constant built in pressure ratio within the greater part of the control range can be brought about by means of a suitable dimensioning of the axial discharge port. The main disadvantage of slide valves is that they are expensive to manufacture in that close tolerances and accurate centering are required. Further, the actuating system which is normally a hydraulic system is also relatively expensive and complicated.
Another solution is to use a rotary type valve wherein the valves are in communication with slots formed in the barrel through which gas is recirculated to suction to create at partial loads. This valve arrangement has the advantage of being less expensive to manufacture than conventional slide valve types, however, the capacity control is not as accurate as with slide valve arrangements. Further, built-in pressure ratio drops with decreasing loads are experienced. Moreover, leakage is obtained across the slots along the rotor bores, particularly at higher loads and at full loads. This shortcoming will be described in greater detail hereinbelow with respect to FIG. 7b. Accordingly, it has been determined that the use of lift valves achieves an economic balance between the need for accurate capacity control as well as the need for minimizing manufacturing costs and operating costs. Lift valves of this type have been known and permit successive compression nodes within the barrel to communicate with one another, thus, effectively reducing the capacity of the compressor. One such valve is disclosed in U.S. Pat. No. 4,453,900 issued Jun. 12, 1984. Further, such valves may communicate an overlying compression node with a recirculation passage which returns pressurized fluid to the suction side of the compressor. However, it is noted that the opening of the lift valve is directly dependent upon the compression spring as well as the internal pressure of the compressor. However, the actuation of such valves is unreliable due to friction, corrosion and other environmental factors which often degradate the positioning of this type of lift valve. Further, while the face of the valve element takes on the approximate shape of the barrel, the valve element is separately formed by casting or other process within predetermined tolerances. In order to economically manufacture such valve elements, the tolerances must be some what relaxed which may result in the leakage of pressurized fluid between compression chambers thereby degrading the efficiency of the compressor.
Clearly there is a need for an accurately controlled and inexpensively manufactured valve system for controlling the capacity of a oil flooded rotary screw type compressor. Such a valve system to include a plurality of serially positioned lift valves which may be readily manufactured within a zero tolerance, with each when opened reducing the capacity of the compressor a predetermined amount.